Severe Combined Immunodeficiency Disease (SCID) consists of a spectrum of uniformly fatal childhood disorders in which lymphocytes, especially T cells, are absent or severely defective in function. Our recent work indicates that abnormal microtubule and membrane capping properties are associated with several different forms of SCID. We plan to generalize this evidence by studies of microtubule and capping properties in new SCID patients and to extend it by studies of SCID patients during immunological reconstitution due to successful thymus or bone marrow transplantation. The first complete dissection of normal human lymphocyte microtubule and capping properties will emerge from parallel analyses in maturation and functional subsets of both thymus and peripheral blood T cells. These data will provide a reference for the studies in SCID. They will also reveal morphological, membrane and cytoskeletal changes accompanying normal lymphocyte differentiation. In addition, we will explore the molecular defect in two specific forms of the disease. A rare SCID patient had apparently mature yet functionally inactive T cells. In these lymphocytes, the normal inverse relationship of microtubule assembly to surface capping of Concanavalin A did not apply. Experiments are proposed to investigate microtubule and membrane properties whose impairment might be linked to the functional defect of these cells. In other, more common, forms of SCID, immune dysfunction is linked to the absence of enzymes of purine metabolism, particularly purine nucleoside phosphorylase and adenosine deaminase. We will explore our recent, remarkable observation that a portion of T cell purine nucleoside phosphorylase exists in a detergent-insoluble form in association with centrioles. In particular, we hope to determine if abnormal centriole replication or impaired centriolar function in the organization of cytoplasmic and mitotic microtubules may underlie the failure of lymphocyte maturation in SCID linked to defective purine metabolism.